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Simple LED circuit.. HELP!

Discussion in 'Misc Electronics' started by [email protected], Nov 11, 2005.

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  1. Guest

    I am having problems with this one, and it shouldnt be that hard...

    I have a 9v battery, and I want an LED to be on above 6.5 volts and off
    below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
    off at 6.4...

    How do I do this?!
  2. Don Taylor

    Don Taylor Guest

    One way might be to include an op-amp and a few resistors to your

    If you google for
    circuit op amp hystresis
    you can find some examples. The idea is that using a few resistors
    you can make the op amp output switch from low to high at a fairly
    specific voltage. You would probably use two resistors to create a
    voltage divider to make your 6.5 volts. Then you would use two more
    resistors to create the feedback needed to get the op amp to turn on
    and off at that voltage. Then you would use the output of your op
    amp to drive your LED and resistor.
  3. CWatters

    CWatters Guest

    For that level of accuracy you need something like a voltage comparator.
  4. DaveM

    DaveM Guest

    View this with a fixed pitch font such as Courier

    | |
    | ------
    | \ / LED
    | \/
    | -----
    | |
    | |
    | \
    | / 620
    | \
    | /
    | |
    | 10K |\ |
    +--/\/\/\/\-----+-----| \ |
    | | - | \ ______|
    \ | | /
    1.5 / +-------------| / LM339 or similar comparator
    K \ | | + |/
    / | |
    | | \
    +-------+ / 27K
    | \
    -----/ /
    / ^ |
    / \ 4.7V |
    --- 400 mw |
    | |
    | |

    Resistors are 1/4W 5%
    If you want to adjust to exactly 6.5 V for switching, replace the 10K
    resistor with a 7.5K fixed resistor in series with a 5K trim pot. Caveat...
    this circuit does not provide hysteresis, so if the source is a bit noisy or
    fluctuates around the switching point, the LED may flicker on/off. But
    this certainly meets your original spec.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in
    the address)

    Never take a laxative and a sleeping pill at the same time!!
  5. Google for "window comparator."
  6. Guest

    Alright, thanks for all the info guys! I will look into all your
    solutions this morning. For the record, I am measuring my own supply
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